Antiship torpedo defense system

ABSTRACT

The detection and localization of an antiship torpedo is accomplished by  ustic and optical means. A ship&#39;s sonar determines the approximate location of the incoming torpedo and a laser scans this location with energy in the blue-green spectrum. Surface reflections are gated out of a linear detector array and reflected portions of the scanning beam which penetrate the water&#39;s surface are divided among the elements of the linear detector array into field of view element signals. These field of view element signals are subsequently compared in time and amplitude to determined disparities between them which point to the torpedo&#39;s position, depth and bearing. Optionally, spectral and polarizing filters are provided to enhance the signal to noise ratios and to facilitate signal processing.

BACKGROUND OF THE INVENTION

Antiship torpedoes constitute a danger to the ships of nations dependingon imports or relying heavily on foreign commerce. They are the majorthreat to the noncombatant ships, particularly, where they areunescorted and vast amounts of war materials must be rapidly deliveredto stem a confrontation.

Therefore, it becomes imperative that systems be developed for detectingand neutralizing torpedoes.

Straight running torpedoes, as opposed to being homing or guided,typically are dodged by sharply maneuvering a ship out of their paths.Usually, early detection or an awareness that a torpedo has been firedis needed so that random evasive action can be taken. This Russianroulette approach sometimes works, sometimes doesn't.

While studies indicate that a certain percentage of ships escape damageby the maneuvering exercise outlined above, blind turns do not deceivehoming or guided torpedoes. Some acoustic homing torpedoes can be jammedby some acoustic noise jammers; yet, these jammers don't affect straightrunning torpedoes nor the signature homing torpedoes. Particularly withrespect to the latter, synthesizing or masking a ship's signature is acomplicated, expensive procedure and the results have fallen short ofexpectations.

Ship-length barriers have been proposed. These envelope a ship in aprotective mesh or other suitable barrier like material and are eithercontinuously dragged along in the water or are thrown overboard whenthere is a threat. Even in the case when the barriers are to be deployedafter an antiship torpedo has been detected, the barriers tend to beponderous and compromised ships' performance by slowing them down andincreasing fuel consumption. Furthermore, after a torpedo has beendetected, ballistic missiles, high-velocity, flat trajectory missiles,barriers of hovering charges with proximity fuses or hedgehog depthbombs are marginal since contemporary acoustic ranging techniques failto provide a precise location of an incoming torpedo. Detection andtracking of the oncoming torpedoes by the latest towed tandem arraysleaves much to be desired for reliable interception.

Because of the limitations of the acoustic detectors, optical antishiptorpedo detection systems are being investigated. A light detection andranging (LIDAR) system has been proposed for detecting shallow submergedobjects so that a hydrofoil boat, for example, could take evasiveaction. A paper entitled "Grazing Angle LIDAR for Detection of ShallowSubmerged Objects" was presented at the Dec. 11, 1978, meeting of theOptical Society-IEEE Laser Conference in Florida by Richard D. Anderson,Robert F. Howarth, and Gregory C. Mooradian. The LIDAR system disclosedat the conference, while a meritorious advance in the state-of-the-art,did not meet all expectations even though detection of some near surfaceobjects was obtained by reflections of energy in the blue-greenspectrum. Precise localization needs to be improved and the depth ofdetection needs to be increased to detect torpedoes running at depthsdown to sixty feet. The extraction of the signal from the clutter signalcreated by surface-return and volume-backscatter-seawater-return alsoposed problems which limited its effective depth.

Airborne methods of optically detecting submerged objects have beenattempted. One is disclosed by Elliot H. Kahn in his U.S. Pat. No.3,604,803 and is entitled "Optical Detection Method for SubmergedObjects". He proposes an aircraft flying above the water and verticallydirecting a blue-green laser beam and sensing the reflections to detectthe submerged object. While this method may meet with a certain degreeof success, it cannot be employed in the manner envisioned by the methodof this invention which concerns itself with a ship effecting thedetection of a torpedo at a relatively small grazing angle with respectto the water's surface and the subsequent neutralization of the torpedo.Backscattering in the present instance is of a magnitude far greaterthan that tolerated by the Kahn method.

Thus, there is a continuing need in the state-of-the-art for a methodand system for assuring the detection and location of an antishiptorpedo by the target ship which effectively allows the neutralizationof the torpedo.

SUMMARY OF THE INVENTION

The present invention is directed to providing a method for a ship'sdetecting and localizing of a submerged object. First there is theacoustic determining of the approximate location of the object and ascanning of at least a portion of the object's location with energy inthe blue-green spectrum. Next, there is a gating out of those portionsof the radiated blue-green energy that are reflected from the water'ssurface and a subsequent receiving of the portions of the radiatedblue-green energy that penetrates the water's surface within theobject's location. Next, there is a dividing of the received portions ofthe radiated blue-green energy that penetrate the water's surface intofield of view element signals. Lastly, there is a comparison of thefield of view elements in time and amplitude to determine whether theobject is within a particular field of view element at a particulardepth.

It is a prime object of the instant invention to provide a method fordetecting antiship torpedoes.

Still another object is to provide a method of detection which reliesupon the acoustic and optical detection capabilities of a ship.

Yet another object is to provide a detection method which not onlydetects the torpedo but the torpedo's wake to assure its neutralization.

Still another object is to provide a detection method which has a quickreaction time.

Still another object is to provide a detection method relying upon ablue-green spectrum final localization.

Still another object of the invention is to provide a detection methodin which the apparatus is fully contained on a ship.

Another object is to provide an electrooptical detection method capableof reliably operating at low grazing angles.

These and other objects of the invention will become more readilyapparent from the ensuing specification when takes with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric schematical representation of the inventionoperationally deployed.

FIG. 2 is a block diagramical representation of the invention.

FIG. 3 is a schematic depiction of the method of the claimed invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings, a surface vessel 10 is shown ina head-on bearing and an antiship torpedo 11 is headed toward thevessel. Being a high target value craft, it is provided with an antishiptorpedo defense system 12 to help assure its survival. The principalcomponents of the system are an electrooptical section 13, abeamsteering section 14 and an acoustic transducer section 15.

Experience has demonstrated that it is advisable to keep theelectrooptical section in a protected location on or below deck in orderto safeguard its relatively delicate component parts from vibrations,shocks and temperature changes which might otherwise affect thesection's performance. Although the state-of-the-art is rapdilyadvancing to assure greater reliability where lasers, detectors andtheir associated equipments are concerned, common sense as well as soundengineering practices dictate it is wiser to prevent unnecessaryexposure of the electrooptical section, particularly in the marineenvironment.

The beamsteering section is mounted in the superstructure about thirtymeters above the water's surface and includes a mirror 16 articulatedand rotated by a servo drive control 17. The drive control iselectronically coupled to the electrooptical section so as to directradiated beams from the electrooptical section to a designated targetarea and to receive reflections indicative, for example, of the antishiptorpedo 11.

The hull mounted or towed acoustic transducer section 15 first detectsand approximately locates an incoming antiship torpedo by either activeor passive means. The bearing resolution within a maximum two thousandmeter range is about plus and minus five degrees. Since the transducersection is in all likelihood an array, range can be approximated.

Once the approximate location of the torpedo is acoustically located,the electrooptical section 13 proceeds to optically determine thetorpedo's exact bearing, range and depth. A computer clock 18 isresponsive to signals from the acoustic transducer array to actuate theservo drive control 17 which properly orients mirror 16 so that anoptical beam will illuminate the approximate area where the incomingtorpedo is. Control signals from the computer clock are fed to a pulsedvoltage source 19 which drives a laser 20. The laser emits tennanosecond pulses of blue-green light at a two hundred pulse per secondrepetition rate. Each pulse has about fifty millijoules power. Thepulsed light beam is reflected from a mirror 21 to a beamsplitter 22 andonto the surface of mirror 16 which directs the beams onto theapproximate torpedo area.

The divergence of the pulsed beam of blue-green light is about onemilliradian. Thus, the projected beam is circular in cross section butwhen it impinges on the water's surface it assumes anelliptically-shaped optical footprint 23 covering the approximate areawithin which the torpedo is traveling. With the pulsed repetition rateand power mentioned above, the torpedo's detection depth is aboutforty-five meters at a lateral range of about one hundred twenty metersto zero meters at a lateral range of about two-thousand meters.

A torpedo traveling at an anticipated attacking depth, either straightrunning, acoustic homing or signature homing, will be detectable, withinfifteen-hundred meters by the system of this invention. At this range amaximum beam elevation angle θ, sometimes referred to as a grazingangle, is equal to about eighty-eight and one-half degrees, see FIG. 1.In this regard, the wake of the torpedo is more detectable than thetorpedo itself. The bubbles created by cavitation are better reflectorsthan the torpedo body.

Light reflected from the elliptically shaped radiated area is reflectedfrom mirror 15 and back onto beamsplitter 22. The reflected energyimpinges on a mirror 24 and through a vertical polarizer 25. Thepolarizer is oriented to have its axis of polarization oriented in avertical plane to reduce the transmission of energy attributed tounwanted surface reflections, e.g. sunlight and other sources.

A spectral interference filter 26 has a small bandpass to pass thereflected energy in the blue-green spectrum and thereby eliminate a gooddeal of the background noise that otherwise might be present.

Because the first energy reflected from the optical footprint area 23 islargely surface reflections of ambient light and the pulsed blue-greenenergy, an electrooptical shutter 27 blanks or gates this energy outfrom the optical sensor in the electrooptical section. An electroopticalshutter, for example, of the type disclosed in a U.S. Patent andTrademark Office issued U.S. Pat. No. 4,243,300 and entitled "LargeAperture Phased Element Modulator/Antenna" can be selected. This devicecould be used to prevent incident energy from being transmitted when arange gating circuit 28 properly actuates the shutter. The commandsignal for the gating circuitry originates in computer clock 18.

At a time slot determined by the computer clock, the shutter is actuatedto pass energy in the blue-green spectrum that has been refracted intothe water, reflected from the torpedo or its wake and reflected back tothe ship.

A linear detector array 29 is made up of at least fifteen juxtaposedhybrid photo multipliers or silicon diodes, each having a field of view(FOV) of approximately one to three degrees, to receive the incidentenergy. A lens arrangement is optional to assure a uniform FOV for allelements. The adjacent elements in the detector have overlapping fieldsof view so that when reflected energy indicative of an approachingtorpedo is within the approximate location covered by optical footprint23, a comparison can be made in a comparator 30 to determine exactlywhere the torpedo is within the optical footprint. The linear detectorarray and the comparator are fabricated from components well known tothose versed in the electrooptical arts and an explanation of theirexact constituency is not felt to be necessary at this time to avoidbelaboring the obvious. Bear in mind, however, that it is essential tothe successful operation of the method of this apparatus that the fieldof view element signals originate from the discrete photo multipliers orsilicon diodes before a comparison of their relative magnitudes is madein comparator 30. This allows the comparison of optical signal strengthsthat are indicative of depth (bearing and range are obtained from theservo control 17 driving mirror 16).

Operation of the system progresses as follows. First the transducerarray 15 acoustically senses the presence of a torpedo in an approximatelocation. The computer clock 18, in response to the transducer's sensingof the torpedo, actuates pulsed voltage source 19 which in turn driveslaser 20 to emit a series of ten nanosecond pulses of blue-green lightat a pulse repetition frequency of about 200 pulses per second. Theoptical footprint 23 through which the torpedo is passing reflects lightenergy from ambient sources as well as from the surface of the waterback to mirror 16 and into the electrooptic section 13. Since the rangehas been approximately determined by the transducer, range gatingsignals from computer clock 18 are fed to an electrooptical shutter 27via range gate 28 to blind the linear detector array 29 during this timeinterval. During the time that it takes for the radiated pulse beam topenetrate the water's surface, be refracted downwardly onto the targetand reflected back to the ship, electrooptical shutter 27 opens to passthis reflected energy through to linear detector array 29. The FOV ofthe detector element create discrete signals representative of thereflected energy. These are fed signals to comparator 30 wherecomparisons of the magnitudes of signals strengths are made. If a targetis present, some of the signals coming from certain ones of the elementsin the linear detector array have a magnitude greater than others. Thisindicates the presence of a target within a particular location in theoptical footprint. This information can be read out from comparator 30to associated computer circuitry to arrive at a fire control solutionwhich effects the launching of a guided torpedo or air/water interfaceinterception missile.

By means of range gating the detector and the electrooptical shutter,the depth of the incoming torpedo is determined. By means of gryoswithin the servo drive control unit 17 bearing is determined. By meansof angle depression of mirror 16 the servo drive control 17 provides anindication of range.

Looking now to the method or ship's detecting and localizing an incomingtorpedo there is first, an acoustically determining 31 of the locationof the incoming torpedo. Next, thre is a radiating 32 by pulsing 33 ablue-green laser and angularly downwardly scanning 34 the incomingtorpedo's location. A gating-out 35 of those portions of the radiatedblue-green energy that are reflected from the water's surface assuresthe receiving 36 of portions of the radiated blue-green energy thatpenetrates the water's surface within the object's location. A providing37 of vertically polarizing filters helps improve the signal to noiseratio by the elimination of unwanted surface reflections and a filtering38 further improves the signal to noise ratio by passing only the energyin the blue-green spectrum. A dividing 39 of the received portions ofradiated blue-green energy that penetrates the water's surface intofield of view element signals is achieved by there being a positioning40 of a linear detector array to receive the blue-green energy signalsand a separating of the signals into discrete field of view elementsignals. Lastly, there is a comparing 41 of the different field of viewof element signals in time and amplitude to determine whether the objectis within a particular field of view element at a particular depth and,therefore, to localize the position of the incoming torpedo and enableits neutralization.

Obviously, many other modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A method for a ship's detecting and localizing asubmerged object comprising:acoustically determining the approximatelocation of the object by projecting acoustic energy from a location inthe water and through the water; pulsing at least a portion of theobject's location with energy in the blue-green spectrum from a positionout of the water, to the water and into it; angularly downwardlyscanning blue-green energy pulses at a near grazing angle of up tonearly eighty-eight and one-half degrees in the area of the object'slocation; gating-out those portions of the pulsed blue-green energy thatare reflected from the water's surface; receiving the portions of thepulsed blue-green energy that penetrate the water's surface within theobject's location; dividing the received portions of pulsed blue-greenenergy that penetrate the water's surface into field of view elementsignals; and comparing the field of view element signals in time andamplitude to determine whether the object is within a particular fieldof view element at a particular depth.
 2. A method according to claim 1further including:filtering the received portions of the radiatedblue-green energy to enable the passage of the blue-green spectrum.
 3. Amethod according to claim 2 in which the step of dividing includes thestep of positioning a linear detector array to receive the blue-greenenergy signals and separating them into discrete field of view elementsignals.
 4. A method according to claim 1 in which the step of dividingincludes the step of positioning a linear detector array to receive theblue-green energy signals and separating them into discrete field ofview element signals.
 5. A method according to claim 4 in which the stepof dividing is prior to the step of comparing to obtain relative opticalsignal strengths among the field of view elements signals so thatindication of the object's location can be made.
 6. A method accordingto claim 1 in which the step of dividing is prior to the step ofcomparing to obtain relative optical signal strengths among the field ofview element signals so that indication of the object's location can bemade.
 7. A method according to claim 1 further including:providing avertically polarizing filter to reduce the magnitude of the receivedportions of radiated blue-green energy.
 8. A method for a ship'sdetecting and localizing a submerged object comprising:acousticallydetermining the approximate location of the object; radiating at least aportion of the object3 s location with energy in the blue-greenspectrum, the step of radiating being the pulsing of a laser to radiateblue-green energy pulses toward the object's location and the angularlydownwardly scanning of blue-green energy pulses at a near grazing anglein the area of the object's location, the scanning is performed by arotating mirror gating-out those portions of the radiated blue-greenenergy that are reflected from the water's surface; receiving theportions of the radiated blue-green energy that penetrate the water'ssurface within the object's location; dividing the received portions ofradiated blue-green energy that penetrate the water's surface into fieldof view element signals; and comparing the field of view element signalsin time and amplitude to determine whether the object is within aparticular field of view element at a particular depth.